Builders seeking performance beyond the realm of bolt-on supercharger and turbocharger kits— exceeding about 10 pounds of boost, or so—likely need to consider the construction of a custom engine assembly designed specifically for forced induction. In the simplest terms, that means replacing the factory cast rotating parts with premium, forged components; ensuring greater head-clamping power and optimizing the compression ratio.
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Even vehicles with bolt-on forced-induction systems benefit from a purpose-built engine that supports the power adder, as the engine will likely offer greater durability, resistance to detonation, and more overall power. Although any engine buildup is not regarded as inexpensive by most builders or enthusiasts, there are methods to simplify the process and keep the overall cost to a minimum.
The seemingly easiest and leastexpensive option is simply upgrading the vehicle’s existing engine with a forged rotating assembly and boostcompatible, lower-compression pistons. Of course, choosing this option or a more extensive engine buildup requires the removal and disassembly of the original engine. In other words, with the heavy lifting required to remove the engine, making the investment in the engine is better justified in the long run.
It’s important to keep in mind that while GM’s LS engines are commendably robust, durable, and reliable, only the recent LS9 and LSA versions were designed explicitly for supercharging—and all of the engines were tested and validated to perform within carefully engineered parameters. That means, for example, the rods and pistons of the LS7 engine are designed to deliver the 505 rated horsepower within the stated RPM range with a small percentage buffer, but strong as that engine may be, its components were not validated for forced induction.
Building in strength and durability is paramount in the engine’s overall success and longevity. The tremendous power gain delivered by the supercharger or turbocharger lessens the effective differences that lower-mass (lighter) components offer on a naturally aspirated engine. That means the instinct to use, for example, lighter-weight pistons to maximize performance isn’t necessarily the correct one, as a heavier forged-alloy piston may slightly increase friction, but ultimately prove stronger under maximum boost. And at, say, 20 pounds of boost, the marginal weight difference won’t be noticed. In other words, using the strongest rotating parts in addition to a strong cylinder block and premium fasteners is worth the few RPM they may sacrifice in the long run in order to ensure optimal cylinder pressure.
Unless you are planning to use the original engine from your vehicle as the starting point, there is almost an unlimited number of options when it comes to selecting an appropriate cylinder block to use as the new engine’s foundation.
Production automotive (and some truck) LS engine blocks are aluminum and reasonably robust for moderate boost pressure. If your plans for the engine exceed the roughly 800-hp range, a high-performance cylinder block is recommended. Although the strength of the block is crucial, the more important factor is the capacity for greater cylinder head clamping through the use of six head bolts per cylinder. Production blocks (including the supercharged LS9) use only four bolts per cylinder; although the LS9 uses larger, 11-mm head bolts versus other LS engines’ 10-mm head bolts.
There are high-performance LS blocks on the market, including the following.
GM Performance Parts LSX Block
Introduced in 2007, the LSX block is designed to support extreme highperformance combinations, especially high-boost engines. GM Performance Parts claims the block can support turbocharged engines making more than 2,000 hp and more than 20 pounds of boost. This is due largely to the provision for six bolts per cylinder.
The LSX block has a siamese-bore design, with 3.99-inch bores that must be finished to 4.00 inches—with a 4.25-inch recommended maximum bore. The maximum stroke can reach 4.25 inches, but rotating-assembly interference on the cylinder must be taken into account for strokes greater than 4.125 inches. It is offered in a production-style standard deck height (delivered .020-inch taller for machining purposes); and a tall-deck version with a 9.70-inch height.
In the LSX block’s favor is its sturdy design, machining flexibility, six-bolt head-clamping strength, the availability of high-flow heads, and a very low retail price. Working against the LSX is the extra weight of an iron casting versus a production aluminum block.
GM Performance Parts C5R Race Block
When the only other choices for engine builders were production blocks, many turned to the unique C5R racing block that GM developed for its factory-backed Corvette racing team. It afforded a 427-ci displacement, and the block was considerably stronger than production blocks.
Although it makes a great foundation for moderately powered engines, it’s not optimal for higherboost combinations. That’s because the C5R block was designed to support 500 to 600 naturally aspirated horsepower. Most notably, it does not offer six-bolts-per-cylinder clamping. It is also expensive, although prices have come down in recent years.
The C5R is a wonderful piece of engine exotica, but the other cylinder blocks described in this section are better suited to supercharged and turbocharged applications.
Racing Head Service LS Race Block
New in 2009, the LS Race Block from Racing Head Service (RHS) is targeted at maximum-performance combinations, including forced induction. Like the LSX block, it features six-bolt head clamping, including a thick .750-inch deck. In fact, the head-bolt pattern is the same as on the GM Performance Parts LSX block, allowing great interchangeability with cylinder heads. Anything that fits the GM block fits the RHS Race Block. However, the LS Race Block is a lightweight, all-aluminum casting.
The block features a siamese bore design, but with pressed-in, spun-cast-iron cylinder liners. It is available with a minimum 4.125- inch-bore diameter and up to 4.165-inch bores. Both production and tall-deck 9.750-inch versions are available. RHS also touts the LS Race Block as “long-arm friendly,” with a raised camshaft centerline and outboard priority main oiling that enables greater rod clearance. A maximum stroke of 4.600 inches is achievable, delivering more than 500 ci with 4.165-inch bores Big displacement capability, interchangeability with GM LSX heads, and aluminum construction are the LS Race Block’s highlights. A comparatively high price is the only real negative.
World Products Warhawk LS7X Block
Long Island, New York–based World Products was the first to market with a high-performance, six-bolt, aftermarket LS cylinder block—beating even GM Performance Parts’ LSX block.
As with RHS’ Race Block, World’s Warhawk LS7X block is a lightweight aluminum casting. It offers six-bolt head clamping and is available in standard- and tall-deck (9.800-inch) versions. Billet steel main caps are standard and it weighs only about 135 pounds with the main caps installed. Additional details include:
- Range of cylinder bore diameters, from 3.990 inches to 4.115 inches
- Tall-deck version accommodates up to 4.500-inch stroke; standard deck takes up to 4.00-inch stroke, for a maximum displacement of more than 454 ci
- Priority main oiling; the oil circulates to the crankshaft first and the top of the engine is at the end of the oil circuit
- Cast-in provisions for standard small-block Chevy engine mounts, meaning a Warhawk based engine is more easily installed in an older GM car
- Provisions for an external oil gallery that enables greater displacement capability
- O-ring seals on the cylinder liners that prevent hot oil from squeezing between the block and liners, and heating the cylinders
The biggest detractor of the Warhawk block is that its six-bolt head pattern is unique. It is not the same as on the GM LSX or the RHS LS Race Block, meaning the only six-bolt cylinder head options come from World Products. That’s not necessarily a bad thing, as World’s heads offer tremendous flow attributes, but they’re the only six-bolt choices for the block.
Katech Re-Sleeved LS2 Cylinder Block
Katech Performance offers a modified version of the GM LS2 (6.0- liter) aluminum cylinder block. The company removes the stock, 4.000- inch iron cylinder liners and replaces them with larger, 4.125-inch-diameter bores that are also machined at the bottom to accept a 4.000-inchstroke crankshaft. This enables a final displacement of just about 428 ci, or 7.0 liters. The bores can be honed out to 4.130 inches, too, for a 429-ci maximum displacement.
Katech offers the enlarged cylinder block with standard or billet steel main bearing caps. And while the larger displacement of the lightweight aluminum block is desirable, it retains the four-bolts-per-cylinder head-clamping pattern. That means an engine built with this block should be aimed at 1,000 hp or less and/or limited to less than 20 pounds of boost (assuming an allforged rotating assembly).
If building a larger-displacement engine isn’t a main priority for your project, all of GM’s productionbased, four-bolt LS cylinder blocks provide adequate strength for lowand moderate-boost engines.
A forged crankshaft, forged rods, and forged pistons should be the ingredients that comprise the rotating assembly, but there are other factors to consider.
Assuming a new engine build uses a forged-steel crankshaft, it’s important to understand that not all forged crankshafts are created equally. From the factory, only the LS7, LSA, and LS9 engines include a forged crankshaft; all other LS production crankshafts are cast iron.
It is possible to use the LS9 forged crankshaft in other LS engines and brand-new assemblies, but it has a considerably longer snout to support the dry-sump oiling system’s larger, gerotor-type oil pump, as well as a unique flywheel bolt pattern. It is possible to modify the crankshaft to work with other oil pumps and front-engine accessory drive systems, but it is easier and less expensive to spec a forged crankshaft from one of the well-known performance crankshaft manufacturers, such as Callies or Eagle.
But even under the banner of “forged steel,” there are different levels of forgings, based on the materials incorporated with the steel to enhance hardness and durability. The most common forgings used in performance engines are 4130 and 4340. Here’s what those numbers mean:
- The “4” refers to a steel alloy that is mixed with molybdenum for greater overall strength—the more “moly,” the tougher the crankshaft.
- The “1” and “3” numbers refer to other materials mixed in the alloy; the “1” indicates a steel alloy with chromium added, while the “3” in 4340 indicates nickel and chromium are part of the steel alloy, for even greater strength.
- The “30” and “40” numbers refer to the percentage of carbon added to enhance hardness; “30” refers to approximately 30-percent content and “40” indicates an approximate 40-percent content.
While both 4130 and 4340 forged-steel crankshafts are superior to standard cast-iron crankshafts, the 4340 forging is stronger than the 4130 because of its nickel content and higher percentage of carbon. Of course, that greater strength comes with a higher purchase price, but for racing applications it’s worth the investment. A street/strip engine does just fine with a properly prepared 4130 crankshaft.
Proper heat-treating can significantly strengthen the crankshaft, while crankshafts used in engines designed primarily for racing should also be shot-peened for maximum strength. Some builders also have the stress risers on the rod throws removed to improve performance and longevity.
To optimize lubrication, the engine may benefit from slots machined in the crankshaft journals that direct oil at higher RPM. Some racing-engine builders also useful groove bearings to ensure maximum oiling for the rods. Avoid cross drilling the crankshaft, however. While it was a common procedure years ago, most professional builders no longer believe it is effective. In fact, it may do more harm than good in the long run.
The crank-triggered ignition system of the LS engine requires a “reluctor” wheel (also known as a “tone” wheel) mounted on the crankshaft. It’s a toothed wheel that helps determine crankshaft position to ensure spark-timing accuracy. Early LS production engines came with a 24X (24 tooth), while later engines—including all those equipped with electronic throttle control—used a 58X (58-tooth) wheel.
Generally speaking, either wheel can be used on a custom engine build, but selection depends primarily on the engine controller to be used. The more common, later-style GM E38 and E67 controllers support the 58X wheel and electronic throttle control, while earlier LS1A and LS1B controllers support the 24X wheel. The 58X wheel can be used with earlier LS engines and later controllers, but revisions to the camshaft-position sensor requires an LS2/LS3 front cover on LS1/LS6 and some truck engines.
For example, a 24X wheel should be used if you plan to retain the original engine controller on an engine built for a 2002 Trans Am that was originally equipped with the LS1 engine. If, however, you plan to install an LS7 engine and supercharger, the LS7’s 58X wheel must be changed to a 24X wheel if the stock LS1 controller is to be used. Additionally, Lingenfelter Performance Engineering offers a conversion module that allows the 58X wheel to be used with earlier controllers, without the need for sensor or other wiring changes.
Aftermarket, standalone control systems, such as those from F.A.S.T. and ACCEL-DFI, are compatible with either the 24X or 58X wheel.
The two most important factors for pistons in a forced-induction engine are cylinder pressure and strength. Simply stated, the castaluminum pistons of most production LS engines (only the supercharged LS9 comes with forged pistons) are adequate for low-boost, bolt-on power adders, but builders seeking higher power need stronger, forged-aluminum pistons that deliver a lower static compression ratio.
Although commendably lightweight and durable in naturally aspirated applications, the factory cast pistons’ high silicon content makes it rather brittle when compared with a forged-aluminum piston. That brittleness doesn’t stand up well to the excessive pressure generated by the blower or turbo; and it is especially susceptible to damage if detonation occurs.
Forged pistons are manufactured through a process that forms the part by essentially pounding it into shape rather than the poured metal of a cast piston. They are still comprised of alloys, but the manufacturing process brings greater material density and eliminates the chance for porosity, which greatly enhances strength. They’re also more ductile—the opposite of a casting’s brittleness—and they typically resist heat better than cast pistons. The best forgedaluminum pistons suitable for boost have less than 1-percent silicon content. (Production pistons are referred to as hypereutectic because of silicon content greater than 12 percent.)
Generally, there are two grades of high-performance forged-aluminum pistons: 4032 and 2618. The 4032 forgings (which contain a small amount of silicon) are less expensive, but not as strong as silicon-free 2618- forged pistons. If there’s a trade-off with forged pistons, particularly 2618 forgings, it is increased coldstart engine noise due to thermal expansion. The silicon in hypereutectic pistons minimizes the piston’s expansion when the engine warms up, allowing for a much tighter piston-to-cylinder-wall tolerance, but the low silicon content of forged pistons means they “grow” more in the cylinder bore. Consequently, forged pistons need greater pistonto-wall clearance, with 2618 pistons needing the most.
In general, a 4032-forged piston needs approximately .0025- to .0035- inch piston-to-wall clearance, while 2618 pistons need about .0035- to .0045-inch clearance. That extra clearance means forged pistons typically generate an unsettling knocking noise known as piston slap when the engine is cold. The noise goes away as the cylinders and pistons.
heat up, causing the pistons to grow and fill up the space. (If the noise doesn’t abate after the engine warms up, it may indicate an incorrect engine assembly or other, more serious engine problems.)
Other attributes that contribute to a stronger “blower piston” include reinforced pin bosses (the areas on either side of the piston skirt where the pin slides in) and a thick piston crown. That’s the area between the top ring and the top of the piston. A thicker crown better withstands the punishment of detonation, as well as the generally hotter temperature and cylinder pressure that come with a highly boosted engine.
Besides selecting a forgedaluminum design, the pistons for a supercharged or turbocharged engine should be targeted to deliver a compression ratio between 8.5:1 and 9.5:1. This typically means using a D-shaped head with a dish (also known as an inverted dome) or strictly a dished piston and matching it carefully with the projected combustion chamber volume. Most LS production engines came from the factory with relatively high compression ratios, including greater than 10.25:1 (the LS7 engine has 11.0:1 compression). That’s too much compression for a forced-induction engine, making it difficult to prevent detonation.
One more thing: Along with strong, forged pistons, you should also employ heavy-duty piston wrist pins, even at the expense of adding weight to the assembly. As mentioned earlier, the overall weight of a forced-induction engine or its rotating assembly should be secondary to ensuring it is robust enough to withstand the pressure generated by the turbocharger or supercharger. To that end, heavier-yet-stronger wrist pins that are either larger in diameter, or have a thicker wall than those typically used in a naturally aspirated engine, should be considered.
Piston manufacturers such as JE Pistons and Diamond offer a variety of forged applications for LS engines and have excellent technical advisors to guide the builder into selecting the most appropriate parts.
On engines designed for higher boost and higher power levels, the use of ceramic-coated pistons is an effective way to combat excessive cylinder and combustion heat, while also reducing friction. Most piston manufacturers and companies with bearings for high-performance and racing engines offer parts with ceramic coatings. The coatings are generally based on Swain Tech products.
On a piston, a coating on top reduces the heat absorbed by the piston, helping prevent burning or other damage under high-boost and leaner-fuel conditions. A coating on the skirts of the piston reduces heatbuilding friction and the same goes for coated main bearings. These coated parts come at a premium cost over non-coated components, but the hedge against the damage caused by excessive heat makes them wise investments.
Some builders use coated main bearings, too, but this is more of a preventative measure against the possibility of oil starvation, rather than a performance enhancement.
Piston Ring and Ring Pack
Piston rings service the vital job of sealing the cylinders to prevent combustion gases from entering the crankcase, while also controlling oil on the cylinder walls and stabilizing the pistons within the bores. Under the high pressure of supercharger or turbocharger boost, those jobs are all the more important, as maintaining cylinder pressure is essential to performance.
Piston manufacturers that offer “blower” pistons for forced-induction engines generally optimize the ringpack location to provide a generous crown for greater overall strength. But LS pistons nevertheless have a ring pack that is located closer to the crown than, say, old-school smallblock engines. The rings are typically thinner than previous-generation engines, but bring increased stability with reduced friction.
With the higher ring pack and pressure from forced induction, LS piston rings are subjected to significant heat. For the most part, that means using the strongest, most heat resistant rings you can afford. That typically means the top ring is molycoated or similar. Ductile iron has long been the mainstay of rings, but steel is used increasingly for its strength and durability.
Generally speaking, when it comes to ring end gaps, the tighter the gap, the better, as this generally maintains cylinder pressure and resists blow-by longer. Total Seal offers unique, two-piece gapless top and second rings that offer greater resistance to blow-by by preventing a conventional gap from opening between the ring ends.
While production engines’ ring sizes vary, most aftermarket LS pistons are manufactured to support 1.5-/1.5-/3.0-mm rings. Thinner rings can be used to reduce friction, but they are made from specialized material that makes them very expensive. They should only be used in a racing engine that will see repeated disassembly, as thinner rings wear out sooner and require more frequent replacement. Stick with thicker rings for street and street/strip combinations.
One more thing about piston rings: You should make sure they’re available for your desired bore size before ordering the pistons or having the cylinder block machined. Assembly plans go right off the tracks when the pistons arrive and there are no rings to fit them.
The trick to gas porting involves drilling holes strategically in the piston to force the compression ring against the cylinder wall. The idea behind it is that this pressurized ring seal prevents the ring from fluttering at higher RPM, while extending the power curve.
Two types of gas porting are typically used: vertical and horizontal. Vertical gas ports are drilled from the piston deck into the top ring groove and behind the ring. This method is employed more by drag racers. Horizontal gas porting involves drilling holes through the bottom side of the top ring land, extending to the back wall of the ring groove. It is used more in circle track/road racing.
Generally, gas porting is best left to dedicated racing applications, where sustained performance at high RPM delivers the greatest benefit. Also, carbon builds up in the ports, so an engine that does primarily street duty (and does not get regular, between-race teardowns) quickly loses the advantage of gas porting when the ports clog. The pressure on the rings also significantly reduces the ring’s lifespan—another reason to avoid gas porting for street engines.
The higher the expected horsepower, the stronger and beefier the connecting rods need to be. Rod failures typically arise from high RPM strain and/or exhaust-stroke pressure. In general, greater horsepower increases the compressive force on the rods, while greater RPM increases tensile strain. These attributes are amplified considerably with forced induction
Most LS production engines use powdered-metal rods that, like their corresponding cast-aluminum pistons, are surprisingly robust in an unmodified engine. As mentioned earlier, factory engine components are designed to operate in a performance window within a few percent of the advertised horsepower and torque ratings. Consistently pushing beyond that range puts a strain on the internal components they weren’t designed for.
To withstand the strain under boost, high-performance connecting rods need to deliver greater compression strength and tensile strength. The typical upgrade is to a forgedsteel material, such as 4340 steel or 300M. Beyond the greater strength that comes with the denser material, these performance rods are typically thicker in key areas to enhance strength, too.
In most cases, builders choose between I-beam-style and H-beam style connecting rods. Each is known for delivering strength, but each delivers it slightly differently. The I-beam looks more like a conventional connecting rod, but is very thick through the middle, allowing it to handle great compressive loads. H-beam rods have a thin center section, but wide, flat outer sides that provide tremendous stiffness and resistance to bending.
Assuming all other attributes are equal, the I-beam and H-beam offer comparable compressive strength, but the thinner center portion of the Hbeam typically makes it lower in mass than an I-beam. The lighter H-beam design can make more of a difference with primarily street-driven vehicles, where more low-end power is desired.
Problems with performance rods can arise, however, with internal clearance within the cylinder block. Thick, racing-type I-beam rods on larger-stroke combinations (generally, engines greater than 427 ci) can interfere with the bottoms of the cylinders and other walls inside the block. Extreme care must be taken to gently rotate the rod/piston assembly to check for clearance problems. Notching the bottoms of the cylinders, making clearance for other areas within the block, and even machining the small and/or big ends of the rods may be required.
Performance connecting rods and rods used with stroker crankshafts may also cause interference issues with the windage tray. After the rotating assembly moves freely within the block, the windage tray should be installed and the engine carefully and slowly rotated to check for clearance problems. If any of the rods hit the windage tray, washers can be used as shims on the bolt studs. Two or three washers per stud are generally all that’s required to ensure adequate clearance.
4340 vs. 300M and Forged vs. Billet—and Aluminum
The common steel connecting rod forging is made from 4340 steel, which contains up to 2-percent nickel, along with smaller percentages of chromium, silicon, molybdenum, and manganese. It is an extremely durable material for connecting rods, but 300M alloy is gaining favor with many builders. It contains more silicon (approximately 1.5 percent) along with more moly and carbon.
Rods made from 300M can be more expensive, but they are generally stronger than a comparably sized 4340 rod, which enables the manufacturer to downsize the center section by up to 20 percent and still offer the strength of 4340 steel. In a supercharged/turbocharged engine that is already using a number of higher-mass components to reinforce overall strength, the investment in 300M rods can offset a significant source of rotating mass.
Another choice is to choose billetsteel over forged-steel connecting rods. As the name implies, billet rods are cut from a single piece of steel on a CNC machine. This is generally used for custom applications where a manufacturer may only make a few sets of a particular design that wouldn’t be cost effective to set up in a conventional forging operation.
A billet-steel rod can be stronger than a forged-steel rod, but only if the steel used is of higher quality than the 4340 or 300M recipes. Because the material does not have to be as malleable as the steel used in forging, it enables the manufacturer to use very strong steel.
As for forged-aluminum connecting rods, they offer very good strength and the obvious benefit of low mass—an attribute that helps offset the weight of heavy-duty piston and wrist pins. But aluminum rods have only about half the tensile strength of a steel rod and are much more susceptible to stretching and fatigue, so they are typically quite “chunky” in size in order to maintain their shape longer. This can cause cylinder-bore interference problems, requiring machining that could ultimately reduce overall strength. Aluminum rods are also considerably more expensive than forged-steel rods.
Aluminum rods are not recommended for street and street/strip engines. They are suitable for racing engines that will see frequent inspections and teardowns.
LS7/LS9 Titanium Connecting Rods
The titanium connecting rods of the LS7 and LS9 engines are strong and lightweight, enabling very quick RPM buildup, but not necessarily the best option when building a boostready engine. That’s because the rods are designed for the operating parameters of their respective factory engines.
Because the rods are validated to the strength requirements for their respective engines, higher boost and higher horsepower strain their compression-strength resistance. That’s not to say these rods are weak by any measure, but they’re simply not designed for use in, say, a 700-, 800-, or 1,000-hp forced-induction engine.
Sacrificing low-speed RPM capability for the assurance and longevity of a forged-steel rod is a worthy tradeoff.
Pre-Assembled Short-Block or “Crate Engine” Assembly
Several engine builders offer short-block and crate engine assemblies that are targeted at supercharged applications. Starting an engine with one of these can be a cost-effective and time-saving option, as you receive a pre-assembled portion of the engine with a correctly engineered engine base.
Generally speaking, a short-block assembly includes a cylinder block fitted with a crankshaft, rods, and pistons. Typically, there is no oil pump, oil pan, camshaft, or other components. An assembled longblock or crate engine generally adds cylinder heads, camshaft, oil pump, and perhaps oil pan, with other accessories and the induction system left up to you. More complete crate engines generally include an intake manifold and other accessories, such as a water pump.
A good example of ready-to-go short-block assemblies are Katech Performance’s Value 402 (6.6-liter) and Value 427 (7.0-liter) kits. Each is built with a re-sleeved LS2 6.0-liter aluminum cylinder block, a premium 4340-forged-steel crankshaft, forged-aluminum pistons and forged H-beam connecting rods. The components are also balanced and blueprinted during assembly.
The rough cost of the Katech short-blocks is between $6,000 and $7,000, and while that may seem expensive compared to the budget small-block engines many enthusiasts grew up with, the aluminum block and other high-performance parts for the LS engine simply come at a higher price. Nonetheless, one of these short-block assemblies—or a similar assembly from another engine builder— makes a smart starting point for an engine combination.
Written by Barry Kluczyk and Posted with Permission of CarTechBooks